rally tests in user space: why are the admin credentials needed, besides the user ones?

We run the rally tests in user space, with a normal user's credentials and tenant. However, rally still requires the json block

    "admin": {
       "username":
       "password":
      "tenant_name":
 }

besides the user's one

    "users": [
      "username":
       "password":
      "tenant_name":
  ],

Why is that? Is there a way to not specify the admin block?

Thanks in advance!

Regards,

Alessandro